How garden and green waste fired boilers work
Garden and green waste boilers burn shredded tree trimmings, leaves, grass cuttings and park waste collected by municipalities and estates. The fuel is very variable: woody prunings may carry 30 to 40% moisture, fresh grass and leaves 50 to 65%, and the whole stream brings soil, grit and stray plastics. Calorific value is low, often 1,800 to 2,800 kcal/kg as fired. Most plants use a reciprocating or step grate that tolerates oversize and wet fuel, with a tall refractory-lined furnace and preheated undergrate air to dry the fuel on the first grate section. Flue gas passes the boiler, economiser and air heater, then a multiclone and a bag filter, sometimes with lime injection for acid gases, before the ID fan and chimney.
Where the ID fan sits
- Reciprocating grate furnace
- Boiler
- Economiser
- Air heater
- Multiclone
- Lime injection and bag filter
- ID fan
- Chimney
The fan sits after the bag filter, which also collects the lime and reaction products where dry sorbent injection is used. The furnace is kept at a steady suction of a few mmWC so that smoke does not escape from the feed hopper and grate seals when wet fuel flashes off steam. Total suction at the fan inlet includes the boiler, air heater, multiclone and a bag filter that may run at 150 to 200 mmWC on its own, giving 250 to 400 mmWC in all.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with dry woody fuel at full fire |
| Dust at the fan | Low after the bag filter, typically below 50 mg/Nm³; soil silica and fine lime |
| Moisture and dew point | Water vapour of 20 to 30% by volume with wet grass and leaves; water dew point 60 to 70°C |
| Corrosive species | HCl from stray plastics and salted soil; acid condensate on any surface below about 130°C |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for garden and green waste fired boilers
| Blade form | Backward curved, with wear protection at the inlet edges for soil grit |
| Material of construction | Mild steel casing with full insulation; impeller in high strength steel, with SS 316 or corrosion-resistant lining where HCl is significant |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings outside the gas path with a cooling disc |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Casing drain, sealed inspection door, and selection at the wettest seasonal fuel |
What this ID fan has to cope with
Seasonal swing in gas volume
In the rainy season and after lawn mowing, grass and leaf content can double the fuel moisture. Each extra kilogram of water leaves as about 1.2 Nm³ of vapour, so the gas volume at the fan rises sharply while the steam output falls. A fan selected on the dry woody fuel runs out of capacity. The duty is set on the wettest blend, with speed control for the rest of the year.
Acid condensation at shutdown
Green waste brings chloride from plastics, road salt and soil. When the boiler stops, the casing cools through the water and acid dew point, and wet chloride deposits corrode mild steel and pit the impeller. Insulation, a casing drain, purging with dry air before shutdown and corrosion-resistant lining in the worst areas reduce the damage.
Soil grit and bag filter upsets
Soil and sand picked up with the waste are abrasive. Normally the bag filter catches them, but a burst bag or a bypass during start-up sends grit straight to the fan. Inlet edge protection and vibration monitoring that warns of a bag failure protect the impeller.
Energy and control
Green waste plants run continuously, but their output varies with fuel moisture far more than with steam demand. A variable speed ID fan holds furnace pressure without throttling, and saves most when the fuel is dry and the gas volume is low. Drying or storing green waste under cover for a few weeks before firing reduces moisture, steam lost to evaporation, and the gas volume the fan must handle.
ID fan and FD fan in garden and green waste fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the chimney | Before the air heater, feeding the grate |
| What it handles | Wet flue gas with soil grit and chloride | Ambient air from a dusty yard |
| Temperature at the fan | 150 to 180°C, up to 230°C | Ambient |
| Main design concern | Seasonal gas volume and acid condensation | Drying air through a wet, patchy bed |
| Control | Furnace pressure | Steam pressure and oxygen |
See also the Forced Draft (FD) Fan for garden and green waste fired boilers.
Questions buyers ask
Why does the ID fan on a green waste boiler need to be larger than on a wood chip boiler?
Garden and green waste carries much more water, especially grass and leaves, and that water leaves the furnace as vapour. For the same steam output the flue gas volume can be 20 to 40% higher than with seasoned wood chips. The ID fan is sized on this wetter gas, so it is larger in volume for the same boiler rating.
What material is best for a green waste boiler ID fan?
Mild steel is acceptable where the gas stays well above its dew point and chloride is low. Where plastics and salted soil bring chloride, and the plant starts and stops often, an insulated casing and an impeller with stainless steel or corrosion-resistant lining in the wettest areas last much longer. The decision depends on fuel analysis and operating pattern.
How can I reduce ID fan load on a garden and green waste boiler?
Reduce the fuel moisture by stacking and covering the waste before firing, seal air leaks at the feed hopper, ash discharge and air heater, and keep the bag filter differential pressure in its normal range. A variable speed drive then lets the fan follow the lower gas volume rather than throttling against a damper.
Information needed to quote an ID fan for garden and green waste fired boilers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Fuel mix. Proportion of grass, leaves and woody material by season, and the measured or expected moisture range.
- Acid gas treatment. Whether lime or sorbent is injected, and the bag filter design pressure drop.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.